Read-write heads for magnetic disk systems have undergone substantial development during the last few years. In particular, older systems in which a single device was used for both reading and writing, have given way to configurations in which the two functions are performed by different structures. An example of such a read-write head is schematically illustrated in FIG. 1. The magnetic field that ‘writes’ a bit at the surface of recording medium 15 is generated by a flat coil, two of whose windings 14 can be seen in the figure. The magnetic flux generated by the flat coil is concentrated within pole pieces 12 and 13 which, while being connected at a point beyond the top edge of the figure, are separated by small gap 16. Thus, most of the magnetic flux generated by the flat coil passes across this gap with fringing fields extending out for a short distance where the field is still powerful enough to magnetize a small portion of recoding medium 15.
The present invention is directed towards the design of read element 20 which can be seen to be a thin slice of material located between magnetic shields 11 and 12 (12 doing double duty as a pole piece, as just discussed). The principle governing the operation of read sensor 20 is the change of resistivity of certain materials in the presence of a magnetic field (magneto-resistance). Most magnetic materials exhibit anisotropic behavior in that they have a preferred direction along which they are most easily magnetized (known as the easy axis). The magneto-resistance effect manifests itself as a decrease in resistivity when the material is magnetized in a direction perpendicular to the easy axis, said decrease being reduced to zero when magnetization is along the easy axis. Thus, any magnetic field that changes the direction of magnetization in a magneto-resistive material can be detected as a change in resistance.
It is widely known that the magneto-resistance effect can be significantly increased by means of a structure known as a spin valve. The resulting increase (known as Giant magneto-resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of the solid as a whole.
The key elements of a spin valve structure are shown in FIG. 2. In addition to a seed layer 22 on a substrate 21 and a topmost cap layer 27, these key elements are two magnetic layers 24 and 26, separated by a non-magnetic layer 25. The thickness of layer 25 is chosen so that layers 24 and 26 are sufficiently far apart for exchange effects to be negligible (i.e. the layers do not influence each other's magnetic behavior at the atomic level) but are close enough to be within the mean free path of conduction electrons in the material. If, now, layers 24 and 26 are magnetized in opposite directions and a current is passed though them along the direction of magnetization (such as direction 28 in the figure), half the electrons in each layer will be subject to increased scattering while half will be unaffected (to a first approximation). Furthermore, only the unaffected electrons will have mean free paths long enough for them to have a high probability of crossing over from 24 to 26 (or vice versa). However, once these electrons ‘switch sides’, they are immediately subject to increased scattering, thereby becoming unlikely to return to their original side, the overall result being a significant increase in the resistance of the entire structure.
In order to make use of the GMR effect, the direction of magnetization of one of the layers 24 and 26 is permanently fixed, or pinned. In FIG. 2 it is layer 24 that is pinned. Pinning is achieved by first magnetizing the layer (by depositing and/or annealing it in the presence of a magnetic field) and then permanently maintaining the magnetization with an undercoat of a layer of antiferromagnetic material, or AFM, (layer 23 in the figure). Layer 26, by contrast, is a “free layer” whose direction of magnetization can be readily changed by an external field (such as that associated with a bit at the surface 15 of a magnetic disk).
The structure shown in FIG. 2 is referred to as a bottom spin valve because the pinned layer is at the bottom. It is also possible to form a ‘top spin valve’ structure where the pinned layer is deposited after the pinning layer.
Ultra-thin free layers as well as MR ratio are very effective to obtain high output spin valve GMR heads for over 30 Gb/in2 magnetic recording. In general, magneto-resistive devices have a characteristic response curve such that their sensitivity initially increases with the applied field, then is constant with applied field, and then decreases to zero for even higher fields. It is therefore standard to provide a biasing field to keep the sensor operating in the linear range where it is also at its most sensitive. However, as the free layer thickness decreases, it becomes difficult to obtain a controllable bias point, high GMR ratio and good magnetic softness all at the same time. Synthetic antiferromagnets (SyAF) are known to reduce magneto-static fields in a pinned layer, but a large bias point shift due to sense current fields remains a problem for practical use of an ultra-thin free layer. To overcome this problem, the spin-filter spin valve (SFSV) was invented.
In a SFSV, the free layer is placed between the Cu spacer and an additional high-conductance-layer (HCL). SFSV reduces sense current fields in the free layer by shifting the sense current center toward the free layer, resulting in a smaller bias point shift by sense current fields. High GMR ratio is maintained even in the ultra-thin free layer because the HCL improves the mean free path of a spin-up electron while maintaining the mean free path difference between spin-up and spin-down electrons.
As discussed earlier, spin valve GMR heads may be either top or bottom types. The GMR sensor track is defined by a patterned longitudinal biasing layer in the form of two bias stripes. These are permanently magnetized in a direction parallel to the surface. Their purpose is to prevent the formation of multiple magnetic domains in the free layer. The most commonly used longitudinal bias for the bottom spin valve is with contiguous (abutted) junction hard bias. The problem with the abutted junction is the existence of a “dead zone” at the sensor ends. A MR sensor track defined by continuous spacer exchange bias (similar to that for the DSMR) does not have the “dead zone”. This may be critical for a very narrow track for ultra-high density recording application.
A routine search of the prior art was performed. The following references of interest were found. U.S. Pat. No. 5,637,235 (Kim et al.) shows a SV with a capping layer. U.S. Pat. No. 5,896,252 (Kanai) shows a SV with a free magnetic layer composed of a CoFe and NiFe sublayers. while U.S. Pat. No. 5,648,885 (Nishioka et al.) teaches a SV with CoFe free layer.